Fault Tree Analysis system level

Fault-tree analysis This level of analysis concerns critical infrastructures, where multiple conditions are necessary for the systems to ensure its task. This type of approach aims to evaluate the remaining operating capacity (residual operation capacity) of objects such as health-care facilities. The system is broken down into structural, nonstructural, or human components, each one of them being connected with logic operators. 

Fault tree analysis is used primarily as a tool for conducting system or subsystem hazard analyses, even though qualitative or top-level (that is, limited number of tiers or detail) analyses may be used in performing preliminary hazard analyses. Generally, FTA is used to analyze failure of critical items (as determined by a failure mode and effects analysis or other hazard analysis) and other undesirable events capable of producing catastrophic (or otherwise unacceptable) losses. 

The first step in performing a fault tree analysis is to collect the appropriate project description documents, existing hazard analyses, and guidance documents and carefully review them to determine the limits, scope, and ground rules for the FTA.This review includes defining the system to be analyzed, the depth or indenture levels to be included in the effort, and, of course, the nature of the undesired event or failure to be studied. 

A quantitative risk review technique. Cause-consequence analysis is a hlend of fault tree and event tree analysis. This technique combines cause analysis (described by fault trees) and consequence analysis (described by event trees), and hence deductive and inductive analysis is used. The purpose of CCA is to identify chains of events that can result in undesirable consequences. With the probabilities of the various events in the CCA diagram, the probabilities of the various consequences can be calculated, thus establishing the risk level of the system. See also Event Tree Analysis (ETA) Fault Tree Analysis (FTA). 

The SSHA evaluates hazardous conditions, on the subsystem level, which may affect the safe operation of the entire system. In the performance of the SSHA, it is prudent to examine previous analyses that may have been performed such as the preliminary hazard analysis (PHA) and the failure mode and effect analysis (FMEA). Ideally, the SSHA is conducted during the design phase and/or the production phase, as shown in Chapter 3, Figure 3.4. However, as discussed in the example above, an SSHA can also be done during the operation phase, as required, to assist in the identification of hazardous conditions and the analysis of specific subsystems and/or components. In the event of an actual accident or incident investigation, the completed SSHA can be used to assist in the development of a fault tree analysis by providing data on possible contributing fault factors located at the subsystem or component level. 

Where mass limits are the main defense against criticality, assay and inventory records arc the important control measures. Fault tree analysis shows where the most effective point of application for an assay system is to cover several areas with one system. This is done by tracing the source of an overbatch to the lowest level cause, which often is the same for different areas. For complex facilities with many interacting process areas, fault tree analysis can be a most valuable tool of the safety professional. 

Historically the FMEA technique has been used extensively used in the aerospace, automotive, electronics, and defense industries because they all require analysis of complex mechanical systems and because the failure of an equipment item can have such catastrophic consequences. The FMEA method has not been used a great deal in the process industries partly because of a perception that its use is very time consuming. The same criticism is sometimes made of the Fault Tree Analysis. In fact, neither the FMEA or FTA methods need to take a lot of time it is just that the level of detail that is necessary for the analysis of, for instance, a nuclear reactor or airplane wing is much greater than that needed for a... 

Where multiple, diverse hazards exist, the practical approach is to treat each hazard independently, with the intent of achieving acceptable risk levels for all. In the noise and toluene example, the hazards are indeed independent. In complex situations, or when competing solutions to complex systems must be evaluated, the assistance of specialists with knowledge of more sophisticated risk assessment methodologies such as Hazard and Operability Analysis (HAZOP) or Fault Tree Analysis (FTA) may be required. However, for most applications, this author does not recommend that diverse risks be summed through what could be a questionable methodology. 

The primary system safety tools being used are hazard analysis and fault tree analysis. However, the transit industry could very much benefit from more human factors safety analysis. Though the industry has used it before, it has never been applied to the same level of detail as it has in the commercial nuclear power industry or civil aviation. Even though quantitative human factors safety analysis is still controversial, it could prove useful in the transit industry. Some countries, such as Erance, have already started to look more deeply into this. 

FMEA is simply an analysis tool that identifies all the ways a particular component can fail and what its effects would be at the subsystem level and ultimately on the system. FMEA is vastly different from fault tree analysis. Fault tree analysis is a top-down analysis of faults in a system. FMEA is a bottom-up analysis that identifies failures (not necessarily faults) in the system. The fault tree starts with the top-level or system-level concern (top event) and then works down to the events that lead to that top event. FMEA does exactly the opposite it starts with the components in the system and analyzes failures and how they impact the subsystan in which it is housed and what are the propagated effects across the syston. 

Risk analysis is required to evaluate the accident frequency and consequences. In railway industry. Safety Risk Model (SRM) is used to estimate system risk, SRM consists of Fault Tree Analysis (FTA) and Event Tree Analysis (ETA). Fault tree estimates accident frequency considering system failure logic (Muttram 2002). It calculates top event frequency or probability using minimal cut sets. Basic events in fault tree describe the component failures they can model revealed repairable failure, revealed unrepairable failure and unrevealed repairable failure with periodic inspection (Andrews Moss 2002). The above failure models for basic event are not enough to consider the effects of maintenances on risk as these models cannot describe multi-level repairsor inspections in details. 

ISA Standard, Safety Instrumented Functions (SIF)— Safety Integrity Level (SIL) Evaluation Techniques Part 3 Determining the SIL of a SIF via Fault Tree Analysis, TR84.00.02-2002, Part 3, 2002. ISA-The Instrumentation, Systems, and Automation Society. Research Triangle Park, NC. 

The systems risk analysis is performed in two steps. As a first step there is a FMEA and a conventional fault tree analysis. This is done for two reasons. The FMEA is done at a very early point in the project. At this early stage, the system architecture definition is not yet complete. High level errors and possible unsafe states are... 

State machines are also used in the AADL error model annex [20] to describe the failure behavior of components, but in contrast to RCM, these are compiled into generalized stochastic petri nets (GSPNs) for evaluation. Hierarchical abstraction of the failure behavior is applied by summarizing states of the subordinated hierarchy level and building the state machine of the current component in this way. But there is no further guidance or automation regarding abstraction. To the best of our knowledge, we can say that there is no other approach that systemizes hierarchical abstraction of fault tree analysis similar to SCM. 

The INRS Model builds on the principles of fault-tree analysis (Leplat, 1978). The model focuses on variations or deviations from the usual course of work at the work-systems level. There are four classes of variations, those related to the individual, the task, equipment and the environment respectively. We here see a clear relation to ergonomic models of work systems. The findings from an accident investigation are displayed in an analytic tree, showing causal relations. Figure 5.9. It gives a schematic presentation... 

We can then state that a general tactic for mitigating hazards is to use fault tree analysis to show that their maximum probability of occurrence does not exceed that established for their severity level, and that the integrity level of the system software is at least that required for the given severity level. We can state this as a generalized axiom (with variables) as follows. 

Within process industries characterized by large production units and high levels of automation, risk and accident analysis is focused on the avoidance of low-probability events entailing serious consequences for the plant and its environment. Safety analysis is based here on causal or probabilistic models of the accidental chain of events that can serve to identify deficiencies in the design of the plant and its protective system as well as to predict the level of risk involved in an operation. Methods developed are fault tree analysis, MORT (Johnson 1975) and INRS (Leplat Rasmussen 1984). A detailed analysis of the actual, individual incident or failure is performed to identify these possible weak spots in the plant and its operation. It is a common experience that human acts play an important role in such industrial mishaps so, especially after the reactor incident at Three Miles Island in 1979, much effort has been spent on developing suitable predictive tools for the... 

Fault Tree Analysis (FTA) is a formal deductive procedure for determining combinations of component failures and human errors that could result in the occurrence of specified undesired events at the system level (Ang and Tang (1984)). It is a diagrannnatic method used to evaluate the probability of an accident resulting from sequences and combinations of faults and failure events. This method can be used to analyse the vast majority of industrial system reliability problems. FTA is based on the idea that ... 

Faults are analyzed through a graphical representation of causality known as Fault Tree Analysis (FTA). Faults are used to analyze the effect of failures on the system, subsystem, or operating environment (i.e., to facilities, equipment, or personnel). Failures are associated with a quantitative analysis of the design of the system. Hazards are assessed qualitatively, aud must be analyzed and either eliminated or reduced to an acceptable level of risk through a mitigation process. The relationship between faults, failures, and hazards may best be understood as follows not aU faults are failures and not aU failures present a hazard to the system. 

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